Writing and Deleting Magnets With Lasers

Laser light for writing and erasing information: a strong laser pulse disrupts the arrangement of atoms in an alloy and creates magnetic structures. A second, weaker, laser pulse allows the atoms to return to their original lattice sites. (Source: S. Münster, HZDR)

Scientists at the Helmholtz-Zentrum Dresden-Rossen­dorf HZDR together with colleagues from the Helmholtz-Zentrum Berlin HZB and the Univer­sity of Virginia in Charlottes­ville, USA, have found a way to write and delete magnets in an alloy using a laser beam. The rever­sibility of the process opens up new possi­bilities in the fields of material proces­sing, optical tech­nology, and data storage. Researchers of the HZDR studied an alloy of iron and aluminum. It is interes­ting as a proto­type material because subtle changes to its atomic arrange­ment can completely transform its magnetic behavior. “The alloy possesses a highly ordered structure, with layers of iron atoms that are separa­ted by aluminum atomic layers. When a laser beam destroys this order, the iron atoms are brought closer together and begin to behave like magnets,” says HZDR physicist Rantej Bali.

Bali and his team prepared a thin film of the alloy on top of trans­parent magnesia through which a laser beam was shone on the film. When they, together with researchers of the HZB, directed a well-focused laser beam with a pulse of 100 femto­seconds at the alloy, a ferro­magnetic area was formed. Shooting laser pulses at the same area again – this time at reduced laser inten­sity – was then used to delete the magnet. With a single laser pulse at reduced intensity, about half of the previous level of magne­tization was retained, and with a series of laser pulses, the magne­tization dis­appeared altogether. These obser­vations were made at the Bessy II synchro­tron using a micro­scope that deploys soft X-rays to study the magnetic contrast.

Working with a team from the Univer­sity of Virginia in Charlottes­ville, the scientist were able to clarify what happens in the alloy during this process. The simulations of the American col­leagues show that the ferro­magnetic state is formed when the ultra-short laser pulse heats up the thin-film material to the extent that it melts, all the way from the surface to the magnesia interface. As the alloy cools down it enters a super­cooled liquid wherein it remains molten, despite the tempera­ture having dropped below the melting point. This state is reached because of a lack of nuclea­tion sites – micro­scopic loca­tions where the atoms can begin to arrange into a lattice. As the atoms move around in the super­cooled state in search for nuclea­tion sites, the tempera­ture continues to drop. Finally, the atoms in the super­cooled state must form a solid lattice, and like in a game of musical chairs, the iron and aluminum atoms end up being trapped in random positions within the lattice. The process takes only a few nano­seconds and the random arrange­ment of atoms renders a magnet.

The same laser, but with a reduced inten­sity, rearranges the atoms into a well ordered structure. The weaker laser shot melts only thin layers of the film, creating a molten pool sitting on the solid alloy. Within a nano­second after melting, and as soon as the tempera­ture drops below the melting point, the solid part of the film starts to regrow, and the atoms rapidly rearrange from the disordered liquid structure to the crystal lattice. With the lattice already formed and the tempera­ture still being high enough, the atoms possess suf­ficient energy to diffuse through the lattice and separate into layers of iron and aluminum. PhD student Jonathan Ehrler summa­rizes: “To write magnetic areas, we have to melt the material from the surface down to the interface, while to delete it, we only need to melt a fraction of it.”

In further experi­ments, the scientists now want to inves­tigate this process in other ordered alloys. They also want to explore the impact of a combi­nation of several laser beams. Inter­ference effects could be used to generate patterned magnetic materials over large areas. “The remarkably strong changes to the material property may well lead to some interesting appli­cations,” reckons Bali. Lasers are used for many dif­ferent purposes in industry, for instance in material processing. This discovery may also open further avenues in optical and data storage techno­logies. (Source: HZB)

Reference: J. Ehrler et al.: Laser-Rewriteable Ferromagnetism at Thin-Film Surfaces, ACS Appl. Mater. Interfaces, online 17 April 2018; DOI: 10.1021/acsami.8b01190

Link: Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf HZDR, Dresden, Germany

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